This invention relates to a gas absorption system based on a physical scrubbing agent for the separation of hydrogen sulfide and carbon dioxide from a gaseous mixture, said scrubbing agent being a liquid having a higher absorption capacity for hydrogen sulfide than for carbon dioxide, and in particular to a system wherein at least a first stream of scrubbing liquid, loaded with hydrogen sulfide and carbon dioxide, is withdrawn from the scrubbing stage and thereafter separated by regeneration into a hydrogen sulfide containing gaseous phase and into regenerated scrubbing liquid.
A process step frequently required in the processing of raw gas streams is the separation of sour gases which are understood to encompass essentially carbon dioxide, hydrogen sulfide, carbonyl sulfide, hydrocyanic acid, and mercaptans. For one or more reasons, e.g., corrosion or catalyst poisoning, such acidic compounds must be removed prior to downstream treatment of the residual gaseous components. Most frequently found and generally in the highest concentrations of the raw gas streams are carbon dioxide and hydrogen sulfide.
Examples of such raw gas streams include, but are not limited to, natural gas, cracked gases, and especially hydrogen-containing gaseous mixtures. To produce industrially useful hydrogen-rich gaseous mixtures, for example, feed gas for hydrogenations, ammonia synthesis, methanol synthesis, etc., conventional starting materials at the present time are crude oils, refinery residue oils, coal, natural gas or similar carbon-containing substances. These raw materials, which in most cases contain sulfur, are subjected to oxidative thermal cracking with oxygen (partial oxidation) at an elevated temperature. After the separation of entrained solids and liquids, e.g., soot, tar, naphthalenes, higher hydrocarbons, and water, a gaseous mixture is obtained consisting essentially of hydrogen, carbon oxides, and hydrogen sulfide, with possible traces of nitrogen, argon, methane and other impurities. If it is intended to use this gas for oxo synthesis, the sour gases are immediately removed so as to provide a synthesis gas consisting essentially of carbon monoxide and hydrogen. In contrast, if it is intended to obtain a gas consisting essentially only of hydrogen, for example hydrogenation hydrogen, or a feed gas mixture for ammonia synthesis, then the carbon monoxide contained in the gas is subjected to a water-gas shift conversion, resulting in oxidation of carbon monoxide to carbon dioxide and in the formation of additional hydrogen. In such a shift conversion, any mercaptans and any carbonyl sulfide which may be contained in the raw gas are reduced to hydrogen sulfide so that the sour gas to be rcmoved from these gases consists essentially of only carbon dioxide and hydrogen sulfide.
To remove sour gases, physical scrubbing processes (among others) have been utilized for years on a large scale. The absorption liquids used in such physical scrubbing processes absorb the sour gas components without simultaneous chemical reaction and can again be freed of these components by simple expansion, heating and/or distillation. For the separation of carbon dioxide and hydrogen sulfide in particular, polar organic solvents, especially methanol, capable of absorbing considerable quantities of sour gas at temperatures below 0.degree. C., proved to be particularly suitable. In this connection, it proved to be especially advantageous for the Henry's law constants of both components to be different, so that in a physical scrubbing step the differing solubilities of hydrogen sulfide and carbon dioxide in the scrubbing medium can result in the selective scrubbing out of one or the other component.
A process of the aforementioned type, for example, is described in "Linde-Berichte aus Technik und Wissenschaft" [Linde Reports on Science and Technology] No. 33 of May 1973, on page 10. In this process, a hydrogen-containing raw gas is first freed, in a scrubbing column, of sulfur compounds by scrubbing with methanol already previously loaded with carbon dioxide. The resultant scrubbing medium, loaded with carbon dioxide and hydrogen sulfide, after expansion and recycling of the thus-degasified components into the raw gas, is conducted into a hydrogen sulfide enrichment column. In this column a partial regeneration is effected based on the differing solubilities of carbon dioxide and hydrogen sulfide in the methanol; accordingly, a residual gas enriched in carbon dioxide and substantially devoid hydrogen sulfide content exits at the head of the hydrogen sulfide enrichment column whereas from the sump, a methanol stream is obtained containing substantially all the hydrogen sulfide and a much greater ratio of hydrogen sulfide to carbon dioxide than that entering the enrichment column. The methanol stream discharged from the sump of the hydrogen sulfide enrichment column is then treated in a regenerating column so as to form: purified methanol, which is recycled into the scrubbing column; and a hydrogen sulfide fraction.
The hydrogen sulfide fraction, dependent on the sulfur content of the raw gaseous stream to be purified, usually contains between 10% and 70% hydrogen sulfide and carbonyl sulfide; consequently this gas, being an environmental pollutant, must not be simply discharged into the atmosphere; rather, it is usually worked up in a sulfur-recovery plant, for example a Claus plant. Hydrogen sulfide contents of at least 20-30 mol-% are required for the economical operation of such sulfur-recovery plants.
One disadvantage of the above-described conventional process is that a hydrogen sulfide fraction of low concentration is obtained in the purification of gaseous streams low in sulfur. Thus, in those instances where less than a 20 mol-% content of H.sub.2 S is obtained, the Claus plant will be uneconomical. Furthermore, in some practical applications, for example where sulfur containing catalysts are employed to effect the reaction of scrubbed raw gas, e.g., where the scrubbed raw gas is a feed gas for a carbon monoxide shift conversion, for alkylations or for various hydrogenation processes, for example the hydrogenation of coal, it may be necessary to maintain a minimum content of hydrogen sulfide or carbonyl sulfide in the feed gas; otherwise, the danger exists that the sulfur-containing catalyst will release sulfur and lose its activity. It is therefore required in the use of commercially available, sulfur-containing conversion catalysts to provide a content of about 0.2 mol-% of hydrogen sulfide and/or carbonyl sulfide in the gaseous stream. If the value drops below this minimum sulfur content, sulfur compounds must be admixed. This is done most simply by adding hydrogen sulfide obtained from the sour gas scrubbing stage (i.e., the gaseous stream from the regeneration column) and compressed to the feed gas pressure. However, substantial compressor energy is required for this purpose, especially in the recycling of a relatively low concentration hydrogen sulfide fraction.